Metabotropic glutamate receptor 1 acts as a dependence receptor creating a requirement for glutamate to sustain the viability and growth of human melanomas

Metabotropic glutamate 1 (mGlu) receptor has been proposed as a target for the treatment of metastatic melanoma. Studies have demonstrated that inhibiting the release of glutamate (the natural ligand of mGlu1 receptors), results in a decrease of melanoma tumor growth in mGlu1 receptor-expressing melanomas. Here we demonstrate that mGlu1 receptors, which have been previously characterized as oncogenes, also behave like dependence receptors by creating a dependence on glutamate for sustained cell viability. In the mGlu1 receptor-expressing melanoma cell lines SK-MEL-2 (SK2) and SK-MEL-5 (SK5), we show that glutamate is both necessary and sufficient to maintain cell viability, regardless of underlying genetic mutations. Addition of glutamate increased DNA synthesis, whereas removal of glutamate not only suppressed DNA synthesis but also promoted cell death in SK2 and SK5 melanoma cells. Using genetic and pharmacological inhibitors, we established that this effect of glutamate is mediated by the activation of mGlu1 receptors. The stimulatory potential of mGlu1 receptors was further confirmed in vivo in a melanoma cell xenograft model. In this model, subcutaneous injection of SK5 cells with short hairpin RNA-targeted downregulation of mGlu1 receptors resulted in a decrease in the rate of tumor growth relative to control. We also demonstrate for the first time that a selective mGlu1 receptor antagonist JNJ16259685 ((3,4-Dihydro-2H-pyrano[2,3-b]quinolin-7-yl)-(cis-4-methoxycyclohexyl)-methanone) slows SK2 and SK5 melanoma tumor growth in vivo. Taken together, these data suggest that pharmacological inhibition of mGlu1 receptors may be a novel approach for the treatment of metastatic melanoma.

Dietary downregulation of mutant p53 levels via glucose restriction: mechanisms and implications for tumor therapy

The majority of human tumors express mutant forms of p53 at high levels, promoting gain of oncogenic functions and correlating with disease progression, resistance to therapy and unfavorable prognosis. p53 mutant accumulation in tumors is attributed to the ability to evade degradation by the proteasome, the only currently recognized machinery for p53 disruption. We report here that glucose restriction (GR) induces p53 mutant deacetylation, routing it for degradation via autophagy. Depletion of p53 leads, in turn, to robust autophagic activation and to cell death, while expression of degradation-defective mutant p53 blocks autophagy and enables survival to GR. Furthermore, we found that a carbohydrate-free dietetic regimen that lowers the fasting glucose levels blunts p53 mutant expression and oncogenic activity relative to a normal diet in several animal model systems. These findings indicate that the stability of mutant forms of p53 is influenced by the levels of glucose and by dietetic habits. They also unravel the existence of an inhibitory loop between autophagy and mutant p53 that can be exploited therapeutically.

Abstract 66: Corruption of the activity of the mitochondria citrate transporter, CIC, by mutant forms of p53

Introduction. A distinctive feature of cancer cells is the implementation of the synthesis of fatty acids (FA), (lipogenesis), which is instead suppressed in most normal adult tissues. Because metabolites produced via lipogenesis provide redox potential and influence the activity of enzymes involved in transcription regulation, by turning on this pathway tumor cells might enable metabolic and transcriptional adjustments necessary for survival in various stress conditions. A still unanswered question is whether oncogenes expressed in human tumors directly favor the shift towards lipogenesis and, if so, whether this has consequences for proliferation or for therapy. The initial goal of this study was to identify metabolic gene targets regulated by tumor-derived mutant forms of the p53 gene product, whose role in metabolism is currently unknown.

Results. We found that p53 mutants enhance the expression levels of the mitochondria citrate transporter, CIC. CIC is central to lipogenesis as it scaffolds citrate from the mitochondria to the cytosol, yielding acetyl-CoA, in turn necessary for fatty acid synthesis. Data extracted from gene expression databases, confirmed a strong association between high levels of CIC mRNA and p53 mutant-expressing human tumors, suggesting that up-regulation of CIC is a molecular signature for p53 mutations. CIC up-regulation is also a negative prognostic factor, and a predictor of tumor recurrence, thus mirroring the negative prognostic value of p53 mutant positive tumors. We show that the CIC promoter is transcriptionally up-regulated by mutant p53, via a unique mechanism that involves p53 mutant binding to FOXO-1- and to p53-consensus elements contained therein, as well as a physical interaction between these two proteins. Importantly, while in glucose-deprived cells CIC promoter activity is physiologically shut down, p53 mutants maintain higher levels of CIC transcription in these conditions. This is in contrast with wild-type p53 that suppresses CIC transcription. Furthermore, we demonstrate that inhibition of CIC with a siRNA, or blocking export of citrate to the cytoplasm with a CIC inhibitor, lowers fatty acid levels and impairs survival of p53-mutant tumors. This effect is not seen in pseudo-normal or tumor cells that have a wild-type p53 gene, again suggesting specificity for mutant p53. Additional experiments indicate that by promoting cytoplasmic export of citrate, elevated CIC levels might hamper mitochondrial pathways of energy production.

Conclusions. We have identified the first metabolic signature gene regulated by p53 mutants, CIC, and shown that some tumors expressing these proteins depend upon CIC up-regulation and FA synthesis for survival especially in metabolic stress conditions. Furthermore, we have identified strategies for targeting CIC activity, thus opening new opportunities for the treatment of p53 positive tumors.

Note: This abstract was not presented at the AACR 101st Annual Meeting 2010 because the presenter was unable to attend.

Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 101st Annual Meeting of the American Association for Cancer Research; 2010 Apr 17-21; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2010;70(8 Suppl):Abstract nr 66.

Abstract 4833: Glucose restriction induces degradation of p53 mutants via a selective autophagy-mediated pathway

Introduction. More than 50% of human tumors harbor missense mutations of the p53 gene product. In its wild-type conformation p53 is degraded via the proteasome, mainly through the action of the MDM2 E3 ubiquitin ligase. Unlike the wild-type protein, mutant forms of p53 accumulate at high levels and elude proteolysis, in part due to an altered modality of interaction with MDM2. High levels of p53 mutants in tumors appear to correlate with resistance to radio- and chemo-therapy, and are proposed to promote tumor progression. Furthermore, therapeutic interventions aimed at reducing p53 mutant levels have shown anti-tumor activity in vitro and in vivo. The mechanisms that regulate the stability of wild-type p53 are well understood during the DNA damage response. However, p53 can also be stabilized during adaptive metabolic stress conditions, such as during glucose deprivation. The goal of this work was to determine the effects of glucose availability on the oncogenic activity of mutant p53.

Methods. We investigated the effects of glucose availability in tumor cells expressing p53 mutants, in transgenic animals harboring p53 mutant alleles, and in xenograft models of breast cancer. The read-outs of these experiments were p53 stability, cellular proliferation and response to therapeutic agents.

Results. We found that p53 mutant protein levels are exquisitely sensitive to glucose availability: high glucose levels stabilize p53 mutants, while glucose restriction (GR) leads to their MDM2- and proteasome-independent degradation. This degradation requires instead the participation of the autophagic machinery and of the autophagic protein Beclin-1, which forms a complex with p53 in GR-treated cells. Importantly, in breast cancer cell lines GR alone does not lead to a net increase in autophagic flux, thus differing from autophagy activated during serum- or amino acid- depletion. Rather, GR uses the autophagic machinery to degrade selective targets, such as mutant p53. We further show that tumor cells where p53 mutant levels are lower due to GR, have reduced proliferation potential compared to tumors expressing native p53, and can be easily chemo-sensitized, significantly reducing the IC50 of all drugs examined. We then asked whether dietary restriction of glucose affects p53 mutant stability in living organisms. Preliminary results indicate that p53 mutant levels are higher in tissues of animals fed with high glucose-diet, relative to animals fed with a low glucose diet. We are currently investigating whether this change in p53 levels affects proliferation rates.

Conclusions. We propose that GR uses the autophagic machinery to induce degradation of oncogenic mutant p53, and thus interrupts their proliferative signals. Our findings further imply that dietary restriction of glucose would keep p53 mutant activity in check. Finally, GR overcomes the well-known chemo-resistance conveyed by p53 mutations.

Note: This abstract was not presented at the AACR 101st Annual Meeting 2010 because the presenter was unable to attend.

Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 101st Annual Meeting of the American Association for Cancer Research; 2010 Apr 17-21; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2010;70(8 Suppl):Abstract nr 4833.

A new direction for myosin

Members of the myosin superfamily of actin-based motor proteins were previously thought to move only towards the barbed end of the actin filament. In an extraordinary reversal of this dogma, an abundant and widespread unconventional myosin known as myosin VI has recently been shown to move towards the pointed end of the actin filament - the opposite direction of all other characterized myosins. This discovery raises novel and intriguing questions about the molecular mechanisms of reversal and the biological roles of this 'backwards' myosin.